I own an aging iPhone 3gs, and once in a while, it can go wonky. Today I restarted my iPhone, thinking this is good practice. Like how we need to sometimes restart our PC. At that time my phone was connected to my office WIFI network. After restarting, the WIFI didn’t connect automatically, so I thought I had to do it manually. I went in to the settings screen, click connect to the specific WIFI network, and then I got the pop up.
“Unable to join the network…”
I tried it several times, restarting my phone 3 more times, but it was the same time. I even ‘forget network’ and keyed in my password again, but the same thing. It can’t be any fault to the network or the phone since it was working just minutes ago. So what I did was, turn off WIFI on the phone, and then turn it back on. Connect it and it works again. Hhm, weird eh.
Disclaimer: This is not a universal fix, just for wonky phones like mine.